Electronic devices may experience serious operating difficulties when subjected to unintended electromagnetic noise. Such electromagnetic noise, which interferes with the normal operation of the devices, is generally known as EMI. In order to ensure the reliable operation of electronic devices it is desirable that EMI be reduced to a minimum.
The manner in which electromagnetic interference is suppressed is dependent on the nature of the interference. There are two ways undesirable noise can propagate in signal transmission paths: one is differential-mode interference, and the other is common-mode interference.
Differential-mode interference causes the potential of one side of a signal transmission path to be changed with respect to another side. With this type of interference, the interference current path is wholly in the signal transmission path.
Common-mode interference appears between two signal transmission paths and a common reference plane (ground), and causes the potential of both sides of the transmission path to be changed simultaneously and by the same amount relative to the reference plane. Common-mode noise may be caused by an electric (capacitive) or magnetic (inductive) field when interference is induced in both signal transmission paths equally. The noise voltage that is developed is the same in both transmission paths.
Through a process known as common-mode conversion, common-mode noise currents can be converted to differential-mode voltages due to the inherent impedance and capacitance characteristics of the signal transmission paths and the reference plane.
It is well known that noise in signal transmission paths can be reduced by surrounding the electrical conductors of the signal transmission path with a substance capable of absorbing undesired harmonics. Ferrite materials can be used for this purpose.
A typical prior art differential filtering component is constructed as a magnetic core having one or more separately imbedded conductors. However, it is well known that at certain field strengths the core becomes saturated, its permeability reduced, thereby limiting the ability of the core to produce high impedance to undesired noise. When there are more than one conductors imbedded in the same ferrite structure each works as an individual differential inductor with limited mutual coupling. Most of the magnetic flux lines related to the field around one of the conductors will close without encircling other conductors, and any mutual inductance is only due to stray fields. In such a configuration the equivalent circuit consists of differential inductors and common-mode inductors of relatively much smaller value. Such an arrangement results in poor mutual coupling and the structure is suitable mainly for low power differential-mode noise attenuation applications.
In other prior art arrangements, the ferrite core is configured as, for example, an annular ring, with a commonly shared air space for several conductors. The conductors may be wrapped around the core several times in an interleaved fashion to increase mutual coupling. Although such an arrangement may provide high common-mode inductance, this type of core and winding can not be used in circuits requiring high differential inductance. Only leakage inductance, contributed by that part of the magnetic flux not coupled into the other side, produces some differential attenuation. However, this reduces the common-mode effect of the part and forces the designer to compromise of performance. In other words, typical prior art noise filters are generally designed to be effective in applications requiring either a differential-mode inductor or a common-mode inductor, but not both.
Therefore, it is desirable to provide an apparatus which is capable of common-mode noise rejection for pairs of wires carrying differential signals as well as capable of filtering differential-mode noise by in-line inductance.